The standard telephone consists of a base unit and a handset unit connected to each other by an electrical cord. The base unit itself is connected by another cord to a receptacle on a wall, telephone pole or a similar immovable structure to which the telephone network line extends. Therefore, the range of movement of the operator of the telephone is quite limited. Even when the cords connecting the handset unit to the base unit and the base unit to the wall are long, it can be cumbersome to move either the entire telephone around to make calls from different locations or to walk around with the handset unit once a call has been placed. The simple fact that there is always a continuous physical connection between the person making the phone call and the immovable wall or other fixed structure can be a great inconvenience.
The cordless telephone represents a significant improvement over the standard telephone. In the conventional cordless telephone, the base unit is still connected to the receptacle on the immovable wall or the like by a cord so that message signals from the telephone network line may be received and transmitted. However, the handset unit of the cordless telephone is an independently operative unit from which calls may be made and by which calls may be received with no physical connection to the base unit. Instead, the base unit and the handset unit of the cordless telephone communicate with each other over a communication channel established by the transmission and reception of electromagnetic waves, conventionally radio waves. The handset unit may then be taken to distances of up to 300 meters from the base unit, while still making and receiving telephone calls. Since the handset unit is not attached to any fixed power supply, it is generally powered by batteries.
The cordless telephone itself presents certain difficulties. In particular, if several cordless telephones are being used within the same general area, the signal transmitted from one handset unit to its own base unit may be picked up by the base unit of a completely different cordless telephone, leading to crosstalk interference. A second, more sophisticated, difficulty is that, since the radio waves establishing the communication channel between the handset unit and the base unit may be received by any appropriately located antenna, it is possible to "wiretap" the cordless telephone, so that a third party can listen in on or intercept conversations. Under many circumstances, this is highly undesirable. The term "wiretap" is somewhat incongruous in this situation, since there is no physical wire to be tapped, but this term is customarily used for any act of listening in on, or intercepting, other people's telephone conversation, and the present specification will use the term "wiretap" in that sense, that is, to mean the unauthorized listening in on telephone conversations through the reception of the radio waves transmitted between the handset unit and the base unit of a cordless telephone.
To overcome these problems, two systems have been proposed. In the first system, mechanical switches for selecting a code number as an arbitrary combination of four digits, from "0000" to "9999", are provided on both the handset unit and the base unit. These switches are manually set by the operator of the telephone set. If both sets of switches are set to the same code number, the communication channel can be established and telephone messages transmitted. If, however, the switches are set to different code numbers, means are provided within the base unit for cutting off the communication channel. The radio waves transmitted between the handset unit and the base unit include this identifying code information. By this means, interference between one telephone and any other cordless telephone operating in the general neighborhood can be avoided and the possibility of wiretapping can be reduced. However, this system requires four manually actuable switches on each of the handset unit and base unit, which inevitably increases the manufacturing cost. Furthermore, the switches are placed so as to be easily operable from the surfaces of the units. As a result, it is quite likely that the operator may inadvertently change the code number of the handset unit by actuating one of the switches without realizing it. When the code number of the handset unit is thereby different from that of the base unit, communication between the two units is prevented. This difficulty can be removed by resetting the code numbers of both the handset unit and the base unit, but this requires the operator to walk back to the base unit, which is the very thing that a cordless telephone is supposed to avoid.
A second system for overcoming the wiretapping and interference problems, proposed by a person or persons having an obligation of assignment to the assignee of the present invention, is to have a memory within each of the handset unit and the base unit and to store the identifying codes therein. When the operator wants to make an outgoing call, the identifying code is transmitted from the handset unit to the base unit over the communication channel created by the radio waves. The base unit contains means for checking whether the received identifying code is the same as the identifying code stored in its own memory. If the two are the same, the communication channel is kept open and the telephone call proceeds. If not, the communication channel is cut off. Similarly, if an incoming call is received by the base unit over the telephone network line, the base unit transmits its stored identifying code to the handset unit, which in turn compares the received identifying code with the identifying code stored in its own memory. Again, only if the two identifying codes are the same is the communication channel kept open.
However, once again this presents difficulties. As earlier noted, the handset unit must contain its own power supply, and to keep it ready to receive an incoming call a certain power drain will always be present. If the power supply is a dry cell battery, it will quickly be discharged, so that there is the significant inconvenience of frequently replacing the battery, not to mention the additional cost.
If, on the other hand, a rechargeable battery is used, such as an Ni-Cd alkaline battery, it may be recharged by mounting the handset unit on the base unit. During the period of recharging, there needs to be some backup for the memory that stores the identifying code. This may be accomplished by either a backup battery or a backup nonvolatile memory. If a backup battery is used, it must be quite powerful, since the period for recharging the battery is not constant. The handset is then large and heavy. Conversely, if a backup memory is used, the entire system becomes more complicated and requires a much greater manufacturing cost. A third difficulty is that if a third party should ever learn the particular code number stored in the memories of a particular telephone, that telephone may be easily wiretapped. Since there is no provision for changing the code numbers, either the telephone must be replaced or it will continue to be susceptible to being wiretapped.
A third system, disclosed in U.S. patent application Ser. No. 06/696,183, cited above as a related application, is for the base unit to have identifying code generating means for automatically generating one of a plurality of identifying codes upon the mounting of the handset unit upon the base unit, and means for initially transmitting the generated identifying code over the communication channel to the handset unit, wherein it is stored in the memory. In accordance with that invention, the identifying code may be easily and automatically changed each time the handset unit is mounted on the base unit, and the need for backup batteries or a nonvolatile memory is eliminated. It remains important to ensure that the identifying codes generated in one particular telephone are different from the identifying codes generated in any other telephone and further are not predictable from one identifying code to the next.